0. In the “Edit Model Attributes” window, provide absolute zero temperature and universal gas constant (8.314).

1. Define the cavity. The cavity could be an interior surface on a closed shell-like structure or a “bag”. Go to “Tools” -> “Surface” -> “Manager” to create a closed surface. Make sure that the surface’s normal is inward. If not, you should change it – go to “Assign” -> “Element Normal” to flip the normal.

2. Define a reference point, which will be used as the fluid cavity reference point.

3. Define the fluid property and fluid cavity (for the initial step) in the “Interaction” module. Provide ideal gas molecular weight (0.044), and specify molar heat capacity (30), if running Explicit. – The example below is pneumatic fluid.

4. In the “Load” module, make sure you specify the initial temperature or progression, if needed. Otherwise, you will get a “zero or negative mass fluid cavity” error when you run the model. Think of the ideal gas equation – without temperature, the mass couldn’t be calculated.

Example input file:

*Heading
** Job name: pressure_5psig Model name: pressure_5psig
** Generated by: Abaqus/CAE 6.14-1
*Preprint, echo=NO, model=NO, history=NO, contact=NO
**
** PARTS
**
*Part, name=body
*Node
1,           0.,    4.1092391,           0.
2,        1.551,    4.1092391,           0.
3,   1.57593668,   4.09541178,           0.
4,   1.58203626,    4.0856452,           0.
5,   1.61876714,   4.09319639,           0.
6,   1.64749312,   4.27981901,           0.
7,   1.68599999,   4.27687263,           0.
8,   1.68599999,   4.18693876,           0.
9,   1.67130005,   4.17223883,           0.
10,   1.65460002,   4.17223883,           0.
11,   1.65460002,  0.202004194,           0.
12,   1.65686595,  0.178665683,           0.
13,   1.67822409, 0.0697100013,           0.
14,   1.66388607, -0.0177931599,           0.
15,   1.65949416, -0.0254923403,           0.
16,   1.64911366, -0.052131068,           0.

…………………………….

58232, -0.309937149,           0., -0.389943779
58233,  -0.31376785,           0., -0.371824145
*Element, type=S3R
1, 11963, 12393, 13783
2, 14009,  8371, 12601
3, 10622, 10999, 10701
4, 13973, 12239, 13932
5, 12375, 12384,  8309
6, 13012, 13133, 13096
7, 12725, 13857, 12741
8,  8636,  9185,  9697
9, 11563,  8736, 12827
10,  8029, 12062, 12132
11, 10658, 10429, 12900
12, 10669, 13543, 10845
13, 11245, 11465,  8215
14, 11504, 11222, 13829
15, 12289, 12379, 12186
16,  8292, 10787, 13475
17, 12451, 12402, 12876
18,  8374, 12500, 12792
19,  9040,  9551,  9722
20,  8354,  9823, 10116
21, 12820, 13336, 11952
22, 10105, 10019,  8217
23, 10509,  8186, 10644

………………………………….

58392, 53876, 58215, 53875, 57166
58393, 58233, 54237, 57253, 58218
58394, 58221, 58220, 56948, 57061
58395, 56884, 57029, 54298, 58230
58396, 58213, 56879, 54267, 58231
*Node
58234,           0.,           2.,           0.
*Nset, nset=body-RefPt_, internal
58234,
*Nset, nset=Set-2
1,     2,     3,     4,     5,     6,     7,     8,     9,    10,    36,    37,    38,    39,    40,    41
42,    43,    44,    45,    46,    47,    48,    49,    50,    51,    52,    53,    54,    55,    56,    57
58,    59,    60,    61,    62,    63,    64,    65,    66,    67,    68,    69,    70,    71,    72,    73
74,    75,    76,    77,    78,    79,    80,    81,    82,    83,    84,    85,    86,    87,    88,    89

……………………………………..

58197, 58198, 58199, 58200, 58201, 58202, 58203
58204, 58205, 58206, 58207, 58208, 58209, 58210, 58211, 58212, 58213, 58214, 58215, 58216, 58217, 58218, 58219
58220, 58221, 58222, 58223, 58224, 58225, 58226, 58227, 58228, 58229, 58230, 58231, 58232, 58233
*Elset, elset=Set-3, generate
11428,  58396,      1
*Nset, nset=cavity_ref
58234,
*Elset, elset=_cavity_surf_SPOS, internal, generate
1,  58396,      1
*Surface, type=ELEMENT, name=cavity_surf
_cavity_surf_SPOS, SPOS
** Section: steel_80#
*Shell Section, elset=Set-2, material=steel_QT, offset=SNEG
0.0088, 5
** Section: steel_113#
*Shell Section, elset=Set-3, material=steel_body, offset=SNEG
0.01243, 5
*End Part
**
**
** ASSEMBLY
**
*Assembly, name=Assembly
**
*Instance, name=body-1, part=body
*End Instance
**
*Nset, nset=Set-1, instance=body-1
58234,
*Nset, nset=Set-2, instance=body-1
11, 2338, 2339, 2340, 2341, 2342, 2343, 2344, 2345, 2346, 2347, 2348, 2349, 2350, 2351, 2352
2353, 2354, 2355, 2356, 2357, 2358, 2359, 2360, 2361, 2362,
2545, 2546, 2547, 2548, 2549, 2550, 2551, 2552, 2553, 2554, 2555, 2556, 2557, 2558, 2559, 2560
2561, 2562, 2563, 2564, 2565, 2566, 2567, 2568, 2569, 2570, 2571
*Elset, elset=Set-2, instance=body-1
11559, 11691, 11823, 11955, 12087, 12219, 12351, 12483, 12615, 12747, 12879, 13011, 13143, 13275, 13407, 13539
13671, 13803, 13935, 14067, 14199, 14331, 14463, 14595, 14727, 42622, 42623, 42624, 42625, 42626, 42627, 42628
………………………………………
42661, 42662, 42663, 42664, 42665, 42666, 42667, 42668, 42669, 42670, 42671, 42672, 42673, 42674, 42675, 42676
42677, 42678, 42679, 42680, 42681, 42682
*Nset, nset=Set-3, instance=body-1
58234,
*Elset, elset=_Surf-1_SPOS, internal, instance=body-1, generate
1,  58396,      1
*Surface, type=ELEMENT, name=Surf-1
_Surf-1_SPOS, SPOS
*End Assembly
*Amplitude, name=my_smooth_step, definition=SMOOTH STEP
0.,              0.,            0.01,              1.
**
** MATERIALS
**
*Material, name=steel_QT
*Density
0.000733,
*Elastic
3e+07, 0.3
*Plastic
64000.,0.
*Material, name=steel_body
*Density
0.000733,
*Elastic
3e+07, 0.3
*Plastic
60000.,0.
**
** INTERACTION PROPERTIES
**
*Fluid Behavior, name=fluid_cavity
*Molecular Weight
0.044
*Capacity, type=TABULAR
30.,
**
** PHYSICAL CONSTANTS
**
*Physical Constants, absolute zero=0., universal gas=8.314
**
** BOUNDARY CONDITIONS
**
** Name: fixed Type: Symmetry/Antisymmetry/Encastre
*Boundary
Set-2, PINNED
**
** PREDEFINED FIELDS
**
** Name: temperature   Type: Temperature
*Initial Conditions, type=TEMPERATURE
Set-3, 300.
**
** INTERACTIONS
**
** Interaction: my_fluid_cavity
*Fluid Cavity, name=my_fluid_cavity, behavior=fluid_cavity, refnode=body-1.cavity_ref, surface=body-1.cavity_surf,
ambient pressure=14.7, adiabatic

** —————————————————————-
**
** STEP: pressure
**
*Step, name=pressure, nlgeom=YES
*Dynamic, Explicit
, 0.01
*Bulk Viscosity
0.06, 1.2
**
** BOUNDARY CONDITIONS
**
** Name: pressure Type: Fluid cavity pressure
*Boundary, amplitude=my_smooth_step
body-1.cavity_ref, 8, 8, 5.
**
** OUTPUT REQUESTS
**
*Restart, write, number interval=1, time marks=NO
**
** FIELD OUTPUT: F-Output-1
**
*Output, field, variable=PRESELECT, number interval=200
**
** HISTORY OUTPUT: cavity_output
**
*Output, history, frequency=1
*Node Output, nset=body-1.cavity_ref
CVOL, PCAV
**
** HISTORY OUTPUT: H-Output-1
**
*Output, history, variable=PRESELECT
*End Step